The present invention relates to network analyzers, and more particularly to a method of canceling interference effects on measurement results of antenna systems.
Interference often exists in antenna systems. For example in a typical communications environment there may be several base stations within a service area such that energy from one base station may be picked up by the antenna of another, causing confusing results when measurements are performed on a particular antenna of interest.
Current interference cancellation techniques include that disclosed in U.S. Pat. No. 5,642,039 by Donald A. Bradley et al, issued Jun. 24, 1997 and entitled xe2x80x9cHandheld Vector Network Analyzerxe2x80x9d. This patent discloses a method of operating a vector network analyzer in the presence of external signals, i.e., interference, with reduced error. A reference oscillator is dithered over a narrow range of frequencies for each measurement so that interference occurs only over a small frequency range, substantially reducing the possibility of error. This solution is hardware-based, and the interference cancellation effect is not consistent across measurement acquisitions. In fact for some acquisitions it may cancel interference well, but for others large residuals remain.
What is desired is a method of interference cancellation in antenna test measurements that is not hardware dependent and provides consistent and reliable cancellation results efficiently and effectively.
Accordingly the present invention provides interference cancellation in antenna test measurements by acquiring an acquisition at a reflection port of an antenna test instrument in response to an internal signal source, stamping the data acquisition time, and measuring a projection vector that has both a reflection signal component and an interference signal component. Another acquisition at the reflection port is performed without the internal signal source using a limited set of data, and a projection vector that has only the interference signal, if any, is examined to detect whether there is a significant interference signal. If there is significant power in the interference only projection vector, a full acquisition of the interference only signal is made, the data acquisition time is stamped, the interference frequency is measured, an interference phase shift between the two data acquisitions is calculated, an interference vector for the combined reflection/interference vector is predicted using the interference vector and the phase shift, and the predicted interference vector is subtracted from the combined reflection/interference vector to obtain the actual reflection signal for the antenna in the absence of interference.